MAGNETO PLASMA DYNAMIC THRUSTER

MAGNETO PLASMA DYNAMIC THRUSTER

The Magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz Force (a force resulting from the interaction between a magnetic field and an electric current) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet.
Generally, a gaseous fuel is ionized and fed into an acceleration chamber, where the magnetic and electrical fields are created using a power source. The particles are then propelled by the Lorentz force resulting from the interaction between the current flowing through the plasma and the magnetic field (which is either externally applied, or induced by the current) out through the exhaust chamber. Unlike chemical propulsion, there is no combustion of fuel. As with other electric propulsion variations, both specific impulse and thrustincrease with power input, while thrust per watt drops.
There are two main types of MPD thrusters, applied-field and self-field. Applied-field thrusters have magnetic rings surrounding the exhaust chamber to produce the magnetic field, while self-field thrusters have a cathode extending through the middle of the chamber. Applied fields are necessary at lower power levels, where self-field configurations are too weak. Various propellants such as xenon, neon, argon, hydrazine, and lithium have been used, with lithium generally being the best performer.
The name once existed in the Star Wars, now proven to be the most powerful form of electromagnetic propulsion. Magnetoplasmadynamic thruster also called Lorentz Force Accelerator or MPD Arcjet has extreme theoretical capability to convert megawatts of electrical power into thrust, makes it the prime candidate for the next generation missions to Mars, Saturn, asteroids and into deep space with both cosmonauts and robonauts. MPDT creates thrust expelling plasma. So far, theoretically MPDT can process more power and create more thrust than currently available any kind of electric propulsion system.

          

Operating principle:

Magnetoplasmadynamic thruster uses Lorentz force to generate thrust. Generally an ionized fuel is inserted into an acceleration chamber, where magnetic and electric fields are created using power source. Then the ion particles are propelled by Lorentz force resulting from the interaction between current flowing through the plasma and extremely applied magnetic field.

By principle there are two main types of MPD thrusters: Applied field and Self field thrusters. Applied field thrusters have magnetic rings surrounding the exhaust chamber to produce magnetic fields. In Self field thrusters there’s a cathode through the middle of the chamber. Xenon, Neon, Argon, Hydrazine and Lithium are used as propellants, but lithium proven to be the best.

Basically MPD consists of two metal electrodes. Rod shaped cathode at the centre and a cylindrical anode structure that surrounds the cathode. When a high electric arc is applied between the anode and the cathode, the cathode heats up and emits electrons. And the emitting electrons collide and ionize the propellant gas to create plasma. The electric current running through the cathode creates a magnetic field; this self induced magnetic field interacts with electric current flowing from the anode, through plasma to cathode producing a Lorentz force. This force pushes the plasma out of the structure. To produce more thrust external magnetic rings are used that creates magnetic fields to accelerate the plasma discharge.

Irrespective of their interior details, magnetic tensor analysis yields a generic thrust relation:

T = (J^2. µ /4pi) [ln (Ra/Rc) + A]

T= total thrust,

µ= vacuum magnetic permeability,

J= total arc current,

Ra, Rb = effective arc attachment radii,

A= parameter depends upon the finer details of the current attachment patterns on the electrodes,

(A<1)

Advantage and capabilities:

MPDT had demonstrated its capability to provide Specific Impulse (Isp) in the range of 1500 – 8000 sec. and thrust efficiencies 30 – 40% or above. The exhaust velocity range is beyond 110,000 m/s, triple the value of current Xenon based thrusters and 20 times better than liquid rockets. An MPD Rocket can make a manned mission into Mars in just 39 days while it takes more than 8 months for conventional chemical rockets.

Disclaimer:

There are several problems on the commercial operation of MPDT. It requires powers in the range of hundreds or kilowatts, that current solar arrays or radioisotope thermal generator are incapable to produce. One possible option is to use nuclear reactors. But many controversies are there using nuclear reactor in space, while Roscosmos and Kurchatov Institute already announced to develop a megawatt scale nuclear spaceship. Magnetohydrodynamics is an alternative option but with many engineering and commercial challenges.

Once these problems are overcome, MPDT will open a new frontier of space exploration and human’s everlasting urge for ultimate knowledge.

CGI rendering of Princeton University's lithium-fed self-field

 MPD thruster (from Popular Mechanics magazine)

 

 

Research

Research on MPD thrusters has been carried out in the US, the former Soviet Union, Japan, Germany, and Italy. Experimental prototypes were first flown on Soviet spacecraft and, most recently, in 1996, on the Japanese Space Flyer Unit, which demonstrated the successful operation of a quasi-steady pulsed MPD thruster in space. Research at Moscow Aviation Institute, RKK Energiya, National Aerospace Universit , Kharkiv Aviation Institute University of Stuttgart, ISAS, Centrospazio, Alta S.p.A., Osaka University, University of Southern California, Princeton University's Electric Propulsion and Plasma Dynamics Lab (EPPDyL) (where MPD thruster research has continued uninterrupted since 1967), and NASA centers (Jet Propulsion Laboratory and Glenn Research Center), has resolved many problems related to the performance, stability and lifetime of MPD thrusters.
An MPD thruster was tested on board the Japanese Space Flyer Unit as part of EPEX (Electric Propulsion EXperiment) that was launched March 18, 1995 and retrieved by space shuttle mission STS-72 January 20, 1996. To date, it is the only operational MPD thruster to have flown in space as a propulsion system.
Result:

The actual performance of the thruster is somewhat less impressive than predicted, with peak thrust in the range of 20-40 N and peak currents of 30-40kA. These levels of performance more closely approximate those that would be expected from an electro-thermal thruster. This discrepancy was expected, and is caused by the fact that my thruster was operated at atmospheric pressure, whereas  these thrusters are typically operated in near vacuum conditions. The higher operating pressure increased the density of the gas and therefore the rate at which the ions collided with each other, resulting in the loss of momentum. The fact that the thruster's performance was near that predicted for a thermo-electric accelerated was also expected, as the dominant thrust mechanism was the thermal expansion of the propellant gas. Predictably, this corresponded to extensive wear and loss of electrode material due to the extreme temperatures and loss of self-shielding mechanisms provided by low pressure operation.   

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